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Two-dimensional Bose fluids—such as liquid-helium films, or confined ultracold atoms—cannot form a condensate, but become superfluid instead. Frictionless flow, proving superfluid behaviour, has now been observed in an ultracold two-dimensional Bose gas that is stirred with a laser beam.

Statistical correlations between particles play a central role in the study of complex quantum systems. A new study introduces microscopic detection of ultracold molecules and demonstrates the measurement of two-particle correlations.

For several years, researchers have aspired to record in situ images of a quantum fluid in which each underlying quantum particle is detected. This goal has now been achieved: here, fluorescence imaging is reported of strongly interacting bosonic Mott insulators in an optical lattice, with single-atom and single-site resolution. The approach opens up new avenues for the manipulation, analysis and applications of strongly interacting quantum gases on a lattice.

Existing techniques for high-resolution imaging of trapped quantum gases are limited to two-dimensional systems, but the approach described here works in three dimensions by magnifying the quantum gas with matter wave optics.

Ultracold atoms in optical lattices provide a versatile tool to investigate fundamental properties of quantum many-body systems. This paper demonstrates control at the most fundamental level, using a laser beam and microwave field to flip the spin of individual atoms at specific sites of an optical lattice. The technique should enable studies of entropy transport and the quantum dynamics of spin impurities, the implementation of novel cooling schemes, engineering of quantum many-body phases and various quantum information processing applications.

Interacting quantum many-body systems in low dimensions is an active research area in ultra-cold gases. Here, Chomaz et al.study the dimensional crossover of Bose–Einstein condensation and observe the emergence of phase coherence in an ultra-cold quasi-2D Bose gas confined to a flat-bottom trapping potential.

The freedom to manipulate quantum gases with external fields makes them an ideal platform for studying many-body physics. Floquet engineering using time-periodic modulations has greatly expanded the range of accessible models and phenomena.

The connection between the topological properties of the ground state and non-equilibrium dynamics remains obscure. Here, Tarnowski et al. define and measure a linking number between static and dynamical vortices, which directly corresponds to the ground-state Chern number.

By subjecting a Chern insulator to a circular drive, its geometrical and topological properties would be accessible from the spectroscopic response. This prediction is now confirmed in a Floquet topological system realized by ultracold fermionic atoms.

Machine learning can help to identify quantum phase transitions. Here a trained neural network is applied to single-shot density images from a quantum gas experiment, realizing the Haldane model and the Bose–Hubbard model.